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Composites Part B 107 (2016) 182e191

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Composites Part B

journal homepage: www.elsevier .com/locate/compositesb

Theory and validation of optical transmission scanning forquantitative NDE of impact damage in GFRP composites

Anton Khomenko a, b, *, Oleksii Karpenko c, Ermias Koricho a, Mahmoodul Haq a, d,Gary L. Cloud a, Lalita Udpa c

a Composite Vehicle Research Center, 2727 Alliance Drive, MI 48910, USAb General Photonics Corporation, 5228 Edison Avenue, Chino, CA 91710, USAc Department of Electrical and Computer Engineering, Michigan State University, 428 S. Shaw Lane, East Lansing, MI 48823, USAd Department of Civil and Environmental Engineering, Michigan State University, 428 S. Shaw Lane, East Lansing, MI 48823, USA

a r t i c l e i n f o

Article history:Received 16 August 2016Received in revised form17 September 2016Accepted 28 September 2016Available online 29 September 2016

Keywords:Polymer-matrix composites (PMCs)DelaminationOptical properties/techniquesNon-destructive testingOptical transmission scanning

* Corresponding author. General Photonics CorpoChino, CA 91710, USA.

E-mail addresses: khomenka@msu.edu, a(A. Khomenko), karpenko@msu.edu (O. Karpenko), khaqmahmo@egr.msu.edu (M. Haq), cloud@egr.msu.emsu.edu (L. Udpa).

http://dx.doi.org/10.1016/j.compositesb.2016.09.0811359-8368/© 2016 Published by Elsevier Ltd.

a b s t r a c t

Non-destructive evaluation (NDE) techniques that can measure both surface and subsurface defects areof critical importance in evaluating the integrity of GFRP composites. In the present work, opticaltransmission scanning (OTS) technique was proposed for acquiring high-resolution, rapid, and non-contact OT (optical transmittance)-scans of pristine and impacted GFRP samples. Advanced data anal-ysis was developed and implemented to identify the number of delaminations at every scan position. Theresults agreed very well with the actual number and extent of delaminations observed directly from across-section of the specimen. Overall, the presented technique lays the groundwork for cost-effective,non-contact, rapid, and quantitative NDE of GFRP composite structures.

© 2016 Published by Elsevier Ltd.

1. Introduction

Glass fiber reinforced polymer (GFRP) composites are a superiordesign choice for many applications due to themultitude of benefitsthey offer, such as light weight, high specific stiffness, high specificstrength, and good resistance to chemical agents. Combined withdesign flexibility and strategic tailoring of mechanical properties,these key advantages have propelled the wide acceptance of GFRPcomposites in marine, automotive, aerospace, sporting, and con-struction industries [1]. However, the increasing use of advancedmulti-component materials brings along major challenges. GFRPcomposites are vulnerable to flaws during fabrication and operation,which could lead to premature failure of structural components.

The elastic behavior and fracture of GFRP composites largelydepend on the mechanical properties of the fibers and the matrix,the strategic stacking sequence of layers, and the selection of weavepatterns. Anisotropy and mismatch of material properties at the

ration, 5228 Edison Avenue,

nton.s.khomenko@gmail.comoricho@msu.edu (E. Koricho),du (G.L. Cloud), udpal@egr.

interlaminar interfaces are roots for many flaws. For instance, alaminated structure subjected to a low velocity impact, such as atool drop, may develop delaminations between the inner layersthat are not visible on the surface. Hence, non-destructive evalua-tion (NDE) techniques that can measure both surface and subsur-face defects are of critical importance in evaluating the integrity ofGFRP composite structures during their service lives.

Current industrial practice involves the implementation of well-established NDE techniques including ultrasonic testing (UT), X-ray,infrared (IR) or visible range radiation to inspect GFRP structures.Depending on the particular application, each of these methods hasits own advantages and limitations. For instance, phased array UTmay furnish precise information about the location of damage andits spatial distribution inside the GFRP laminate [2,3]. However,phased arrays require coupling with the test specimen, and theycan be rather complex and costly because sophisticated electronicsare needed to adjust the time delays between the piezoelectrictransducers for proper focusing of the wave energy at the defectsite. Hence, relatively simpler systems for immersed and aircoupled UT with a single transducer have been routinely used forNDE of composite structures [4e8].

X-ray computed tomography (CT) can provide detailed imagesof delaminations and other defects in composites [8e12]. However,

Table 1Elastic constants of GFRP laminate.

E11, GPa E22, GPa E33, GPa G13, GPa G23, GPa G12, GPa n13 n23 n12

23.1 23.1 6.9 2.54 2.54 3.2 0.28 0.28 0.36

A. Khomenko et al. / Composites Part B 107 (2016) 182e191 183

X-ray CT uses relatively high levels of ionizing radiation, which canbe dangerous for inspectors. Moreover, chamber volume for X-rayCT severely limits the size of the sample which can be evaluated.

IR thermography is suitable for rapid screening of large com-ponents, but it provides little information about the volumetricdistribution of damage [10,13e15].

Optical methods are generally non-invasive, safe, non-contacting, sensitive, whole-field, and inexpensive. Hence, theyare often used in biomedical applications to evaluate the propertiesof biological tissues [16,17]. Some optical techniques initiallydesigned for medical imaging can be adapted for NDE of GFRPcomposites. Conventional techniques for optical NDE of GFRPlaminates include electronic shearography, digital speckle patterninterferometry (DSPI), digital image correlation (DIC) [10,18], digitalholography (DH), and optical coherence tomography (OCT)[12,19e22]. These approaches can be used for locating defects andstudying the mechanical behavior of GFRP composites. However, alimitation of shearography, DSPI, DH, and DIC techniques is thatthermal or mechanical loading of the test specimen is required tocreate a displacement field and locate structural damage.

Both time domain (TD) and Fourier domain (FD) OCT are basedupon registering back-scattered and back-reflected radiation; andhave been used to study internal structure, defects, and stress inGFRP structures [12,19e22]. Although TD OCT and FD OCT havetheir own advantages, they suffer from common major limitation:the penetration depth inside the GFRP specimens is limited to onlya few millimeters due to very strong scattering inside GFRP com-posite [23,24].

In contrast to OCT, ballistic scanners rely on detecting ballisticphotons transmitted through the tissue [25]. Since glass fibers andmany epoxy resins have good transmission properties in the visiblerange, a similar principle was employed by the authors for NDE ofGFRP composites.

In this work, an optical transmission scanning (OTS) system andan advanced data analysis protocol were developed, experimen-tally implemented, and successfully validated for quantitative NDEof GFRP samples. In its basic form, the proposed OTS system con-sists of a light source (laser diode), a photo detector, and a 2Dtranslation stage. The technique provided high-resolution, rapid,and non-contact OT (optical transmittance) scans. The OTS systemwas used for inspection of pristine GFRP samples and thosedamaged by low velocity impact. The number of delaminationsacross the thickness of the impacted samples as provided by theOTS with advanced data analysis was validated by observing thedamage in a cross-section of the impacted GFRP samples aftercutting them with a diamond saw.

2. Materials and methods

2.1. GFRP samples

2.1.1. Manufacturing of GFRP samplesGFRP composite samples were manufactured using a vacuum-

assisted liquid molding process. The reinforcement was S2-glassplain weave fabric with areal weight of 818 g/m2, namely Shield-Strand® S, provided by Owens Corning. The GFRP samplescomprised eight layers of such fabric stacked at the same angle. Thedistribution medium was Resinflow 60 LDPE/HDPE blend fabricfrom Airtech Advanced Materials Group. The resinwas SC-15, a twopart toughened epoxy obtained from Applied Poleramic. The GFRPplate (508.0� 609.6 mm)wasmanufactured in a 609.6� 914.4 mmaluminum mold with point injection and point venting. After thematerials were placed, the mold was sealed using a vacuum bagand sealant tape, and it was then infused under vacuum at 29 in Hg.The resin-infused panel was cured in a convection oven at 60 �C for

two hours and post-cured at 94 �C for four hours. Finally, impactsamples with dimensions of 100� 100� 4.7 mmwere cut from themanufactured GFRP plate using a diamond saw.

2.1.2. Material properties of GFRP samplesElastic properties of the orthotropic GFRP samples used in

experimental study were determined from tensile tests, and arepresented in Table 1 [26].

The refractive index of manufactured GFRP sample can becalculated using the rule of mixtures for the resin and the fibervolumes [19] as:

ncomposite ¼ nresin$Vresin þ nfiber$Vfiber; (1)

where nresin and nfiber are the refractive indices of resin and fiber,respectively; and Vresin and Vfiber are the volume fractions of resinand fiber, respectively. SC-15 is a combination of bisphenol Adiglycidyl ether resin and cycloaliphatic amine curing agent withweight fractions of ~0.77 (100/130) and ~0.23 (30/130), respectively[27]. Weight fractions can be converted to volume fractions usingthe following relation:

Vepoxy

Vhardener¼

mepoxy$rhardener

mepoxy$rhardenerþmhardener$repoxy

!

mhardener$repoxymepoxy$rhardenerþmhardener$repoxy

! ; (2)

where mepoxy and mhardener are the weight fractions of epoxy andhardener, and repoxy and rhardener are the densities of epoxy andhardener, respectively. Using Equation (2), the Vepoxy and Vhardener

For SC-15 can be calculated as ~0.75 (105/141) and ~0.25 (36/141),respectively. Also, density of SC-15 can be calculated as

rresin ¼ rhardener$Vhardenermhardener

¼ repoxy$Vepoxy

mepoxy, i.e., rresin~1160 kg/m3. The

refractive index of bisphenol A diglycidyl ether resin is nepoxy~1.574[28], and average refractive index of cycloaliphatic amine hardeneris nhardener~1.5 [29]. Hence, according to Equation (1), refractiveindex of uncured SC-15 can be estimated as nresin~1.556. Densityand refractive index of S-glass fiber is 2480e2490 kg/m3 and 1.523,respectively [30]. The weight fraction of resin in manufacturedGFRP composite is 0.365 [31]. Using Equation (2), volume fractionsof resin and glass fibers can be found as ~0.55 and ~0.45, respec-tively. According to Equation (1), the refractive index of resultingGFRP composite can be estimated as ncomposite~1.541. However, ithas to be noted that the refractive index of resulting compositedepends on many factors, such as the inspection wavelength,curing conditions, working environment, to name a few.

The linear attenuation of GFRP composite material wasmeasured with 1.7 mW incident radiation power for 4, 8, and 16layer laminates with the average thickness of 2.6, 4.6, and 9.2 mm,respectively. In general, for collimated monochromatic radiation inhomogeneousmedia, the power of the transmitted radiation can becalculated using the Beer-Lambert law [32]:

Pas ¼ P0$Tas ¼ P0$exp

"�XMi¼1

�miaþmis

�$li

#¼ P0$exp

"�XMi¼1

mi$li

#;

(3)

A. Khomenko et al. / Composites Part B 107 (2016) 182e191184

where Pas is the transmitted radiation power that is attenuated bythe local material in its pristine state; P0 is the incident radiationpower; Tas is the transmission coefficient, which accounts only forabsorption and scattering in the test specimen in the absence ofreflections from its interlaminar interfaces; M is the number ofattenuating species of the material sample; mia, mis, and mi are theabsorption coefficient, scattering coefficient, and linear attenuationcoefficient, respectively; and li is the thickness of ith specie. Thetransmitted radiation power for 4, 8, and 16 layer GFRP laminatescorresponded to 8.99, 5.22, and 1.52 V output from the receivingphotodetector. Thus, from the ratios of the photodetector outputscorresponding to GFRP samples with different thickness, a linearattenuation coefficient m was found to be ~2.7 cm�1. Generally, at agiven incident power and signal to noise ratio (SNR), this valuedetermines the maximum thickness of GFRP composite which canbe evaluated using proposed OTS system.

2.2. Drop-weight impact tests

The drop-weight tests were performed according to the ASTMD7136 standard using an Instron 9250 HV Dynatup machine thatwas equipped with an 88.96 kN load cell impactor, a velocity de-tector, and a pneumatic brake to prevent multiple impacts. Theedges of the GFRP specimen were clamped by pneumaticallyassisted grips. The exposed diameter of the composite plate forimpact loading was 76.2 mm, as per ASTM D7136. A 12.7-mmdiameter hemispherical head impactor was used for the impacttesting. Three GFRP specimens were impacted with 20 J energy forsubsequent NDE by the developed OTS system.

2.3. Optical transmission scanning

2.3.1. Experimental setupThe experimental setup for acquiring non-contact OT-scans

consists of a translation stage that moves the sample, a laser sourcethat illuminates the sample, and a downstream photodetector, asshown in Fig. 1a. Fig. 1b shows the test setup for a cantilever beamwith amid-plane crack that was used as the “standard” specimen asdescribed below. Fig. 1c illustrates how the healthy and impactedplates were examined.

In the present setup, the light source is an iBeam-smart-640slaser diode with 640 nm fundamental wavelength, ~1.5 mm beamdiameter that, and up to 150 mW output power. The transmittedradiation is registered using a DET36A Si detector with350e1100 nmwavelength range, 14 ns rise time and 13 mm2 activearea. The voltage at the output of the photodetector is directlyproportional to registered radiation power. The XY-coordinatestage with stepper motors allows for rapid raster scanning of theGFRP samples with a step size of 0.25 mm. The lateral resolution ofOTS systemwas mainly determined by the beam diameter and waskept at 0.5 mm in all the experiments.

2.3.2. Theory of OTSSince the operating principle of OTS is based upon measuring

the optical transmittance of GFRP samples, it is necessary toestablish a relationship between the severity of damage and the

Rdðj; q;4Þ ¼dNr;sðj; q;4Þ þ dNdðjÞ

NiðjÞ$cosj$du¼

�b$f $NiðjÞ$du4

�$F�j0;n0

$

24Gðjp

co

NiðjÞ$co

level of transmitted radiation power measured with the photode-tector. When the radiation interacts with the material, such effectsas transmission, absorption, chromatic dispersion, diffraction,scattering, reflection, refraction, and conversion can mainly takeplace [33]. Conversion of radiation occurs when the dielectric po-larization of the medium responds nonlinearly to the appliedelectric field, and can be neglected if the medium is fairly linear.Refraction affects the direction of radiation propagation, and can beneglected if the incident radiation is normal to the interface.Diffraction is mostly prominent at the edges and is hardly man-ifested in the bulk of a material. Dispersion effects will be insig-nificant if the radiation is quasi-monochromatic. Note that theselatter three effects influence the direction of radiation propagationrather than the energy or the power of radiation.

Power losses partially occur due to reflection, which happenswhenever there is a mismatch between the refractive indexes oftwo materials (e.g., an interface such as a delamination). In order tocalculate the reflection coefficients for s-polarized (RS) and p-polarized (RP) light at the interface between dielectric materials,the Fresnel equations are used [33]:

RS ¼

��������n1$cosqi � n2$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1�

�n1n2$sinqi

�2s

n1$cosqi þ n2$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1�

�n1n2$sinqi

�2s

��������; (4)

RP ¼

��������n1$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1�

�n1n2$sinqi

�2s

� n2$cosqi

n1$

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1�

�n1n2$sinqi

�2s

þ n2$cosqi

��������; (5)

where qi is the angle of incidence, n1 is the refractive index of thematerial through which the light is reflected; n2 is the refractiveindex of the material through which light is further transmitted.

Equation (4) and Equation (5) hold for specular reflection fromideal mirror-like surfaces, and in the case of normal incidence(qi ¼ 0) they reduce to the following formula:

RSP ¼ RS ¼ RP ¼����n1 � n2n1 þ n2

����2 (6)

For instance, at normal incidence angle, the specular reflectioncoefficient RSP for an air (n1 ¼1)/glass (n2 z 1.5 [30,34]) interface isaround 0.04, meaning that approximately 4% of incident radiationis reflected. However, if the surface is optically rough, such as aninterface of interlaminar delamination, diffuse reflection can takeplace. For example, an analytical model for reflection by roughenedplastic surfaces has been proposed and experimentally validated byTorrance et al. [35]. The model pictured a studied surface as con-sisting of small randomly disposed mirror-like facets. The reflectioncoefficient Rd predicted by the model depended on specularreflection from these facets plus a diffuse component caused bymultiple reflections and internal scattering:

;qpÞsq

35$e�c2$a2 þ a$NiðjÞ$cosj

sj$du; (7)

Fig. 1. Optical transmission scanning: (a) experimental setup; (b) test of the “standard” specimen which has a delamination crack (mode I) created by loading it as a doublecantilever beam according to ASTM D5528; (c) impacted specimen under test.

A. Khomenko et al. / Composites Part B 107 (2016) 182e191 185

where j is the zenith angle of incident radiation; q and 4 are thezenith and the azimuthal angles of reflected flux, respectively; dNr,s

is the specular component of reflected flux; dNr,d is the diffusecomponent of reflected flux; Ni is the radiance of the small source;du is the solid angle of the source; F is the Fresnel reflectance; j0 isthe angle of flux reflected from an elementary facet with a surfacenormal n'; G is a masking and shadowing factor; jp and qp are theprojections of j and q onto the plane determined by the facetnormal and the surface normal; a is the angle at which facet nor-mals are inclined with respect to the normal of the mean surface; fis the area of an elementary facet; a, b and c are scalar constantsthat depend on surface preparation.

Equation (7) shows that the interaction of the radiationwith thematerial is complex; and the reflection coefficient can changedrastically depending on incident angle of radiation, observationangle, surface roughness, and refractive index [35,36]. Since GFRPcomposite refractive index itself depends on many factors,modeling of light propagation in composite material is quite chal-lenging and cumbersome task.

The analysis provided belowoffers guidance to the developmentof a simple and straightforward way to quantify the interior impactdamage in materials, particularly laminated composites that havebeen subjected to impacts.

If normally incident monochromatic and collimated laser radi-ation is passed along a local transect of a fairly linear and homo-geneous medium without considering interfaces such asdelaminations, it seems reasonable to use modified Equation (3)and approximate the radiation transmitted to a downstream de-tector as follows:

Pas ¼ P0$Rac$Tas$Rca; (8)

where Rac and Rca are the reflection coefficients of air-to-compositeand composite-to-air interfaces, respectively.

Any possible changes in registered power Pas defined by Equa-tion (8) can be explained by local variations of fiber/matrix contentand thickness over the extent of a composite test specimen in itspristine condition. This conclusion follows from Equation (3),assuming that the reflection coefficients at the top and bottomsurfaces, Rac and Rca remain constant.

Logic suggests an extension of Equation (8) to determine thepower of the transmitted radiation PT in the presence of defectssuch as delaminations by including reflections at interlaminarinterfaces:

PT ¼ Pas$ð1� R1Þ$ð1� R2Þ$…$ð1� RNÞ ¼ Pas$TR1$TR2$…$TRN;

(9)

where R1, R2, …RN and TR1, TR2, …TRN are the local reflection andtransmission coefficients for each of the N interfaces in the transect

of the sample, respectively. It has to be noted that each reflectionand transmission coefficient takes into account combined lossesat both composite-to-air and air-to-composite interfaces of adelamination.

Actual calculation of PT using Equation (9) is problematicalbecause a map of delaminations with their respective transmissioncoefficients, TR1, TR2, …TRN, is not known a priori, and the deter-mining of it would be forbiddingly counterproductive. Offered hereis a practical solution that replaces the set of unknown trans-mission coefficients by a single “standard” value T:

PT � Pas$TN; (10)

The transmission coefficient T is determined by scanning arepresentative sample containing a single interior delamination.Hence, if delamination is considered to be the main damagemechanism, then Equation (10) can be used to relate the registeredtransmitted power and the number of delaminations.

2.3.3. Determination of a “standard” transmission coefficient TThe value of T was obtained from the OT-scan of a double

cantilever beam (DCB) sample after the mode I interlaminar frac-ture toughness test (as per the ASTM D5528 standard) as shown inFig. 1b. The DCB sample was comprised of eight layers of plain-weave S2 glass with a single fracture-induced delamination be-tween the four upper and four lower laminates. The OT-scan wasacquired with a laser output power of 5.2 mW and a lateral reso-lution of 0.5 mm. The transmission coefficient was calculated as theratio of the transmitted radiation powers PT and Pas averaged overtwo separate 2 cm2 regions of the sample, one region containingthe crack and the other without a crack as shown in Fig. 4b. Thesample was clamped with a fewminiature C-clamps prior to OTS toensure that the crack was closed as tightly as possible. The value ofT was found to be 0.61, apparently owing to the diffuse surface ofthe crack interface.

2.3.4. Robustness of OTSIt should be noted that Equations (8) and (9) strictly hold for

only ballistic photons, which travel from the point-like radiationsource down to the photo-detector in a straight line. However, if acollimated beam with a large diameter propagates through ascattering medium such as a GFRP, the transmitted radiation can bescattered, taking on some sort of angular distribution. In such acase, the size of the delamination/air gap inside the sample, vari-ation of sample thickness, and the distance between the detectorand the output interface might affect the measurement of thetransmitted radiation. In addition, propagation of awide laser beamthrough the edges of delaminations will introduce partial attenu-ation and, possibly, edge diffraction effects. These issues can beaddressed by using a laser with a small beam diameter, or by

Fig. 2. (a) Healthy GFRP sample (no impact); (b) GFRP sample after E ¼ 20 J impact.

A. Khomenko et al. / Composites Part B 107 (2016) 182e191186

installing a diaphragm with a pin hole in front of the detector.In the current OTS setup, the effect of a relatively large beam

footprint (d~1.5 mm) was compensated for in signal processing,which allowed for more accurate detection of delaminationboundaries. It was also noticed that the uncertainties in the de-terminations of the delamination contours were mainly governedby the size of the footprint of the laser beam and the associatedtransition region rather than by uncertainties in the estimation of T,as will be seen in the next section.

Another important consideration is that Equation (10) providesa self-referencing capability for the developed system. That is tosay, the thickness of the composite sample and the delaminationdepth do not affect the transmission coefficient for a single inter-face (delamination) T, because it is defined as the ratio of the ra-diation PT transmitted through the region with the delamination tothe radiation Pas transmitted through the healthy region of thesample.

3. Results and discussion

3.1. Raw OT-scans of GFRP samples

The previously described experimental setup for OTS was usedfor inspection of the GFRP samples shown in Fig. 2. The first samplewas healthy (no impact), and the second one was subjected to lowvelocity impact of 20 J. OT-scans were acquired with a laser outputpower of 5.2 mW and a spatial resolution of 0.5 mm. The distancebetween the bottom side of the sample and the receiver was 5 mm.

Fig. 3. (a) OT-scan of healthy GFRP sample; (b) OT-

The raw results before post-processing are demonstrated in Fig. 3.As seen from Fig. 3a, some areas of the plainweave GFRP plate withno damage had higher transmittance than other areas, which wasapparently caused by thickness variations of the sample and glassfiber irregularities as follows from Equation (3). The transmittedpower Pas was minimal when radiation propagated via clusters offibers. In contrast, the highest Pas corresponded to propagation ofradiation via resin-rich areas. Hence, in the case of the pristineGFRP composite, the amplitude values in the output of the photo-detector fell in a region G0 ¼ [Amax, Amin] which was directly pro-portional to the range of transmitted powers [Pasmax, Pasmin].

The incident power from the laser source, P0 was adjustedbefore each scan such that the maximum amplitude registered bythe photodetector Amax was as close as possible to its saturationlimit of 10 V in order to provide the widest measurement range andthe highest SNR.

3.2. Advanced signal processing for quantitative evaluation ofimpact damage

Owing to the mismatch of material properties at the interfaceswithin the GFRP composite plate, the low velocity impact withE ¼ 20 J resulted in multiple interlaminar delaminations whoseareas increased with depth. In order to quantify the extent andseverity of impact damage from the OT-scans, an advanced signalprocessing procedure was developed to determine the delamina-tion contours as a function of depth. The concept is illustrated inFig. 4. Assuming that delamination is the main damagemechanism,the registered power of radiation transmitted through the samplePT can be divided into discrete amplitude levels depending on thenumber of interfaces below the scan point that affect the beampower. If the transmission coefficient T of a similar GFRP samplewith one delamination in its mid-plane is known and only diffusereflection is assumed, meaning no increase in scattering or ab-sorption is taken into account, the range [Amax, Amin] associatedwith the pristine composite can be simply scaled down to plotdamage contours representing N delaminations. The scaling factorcan be established through the use of Equation (10) as [37]:

GN ¼ G0$TN ¼ ½Amax;Amin�$TN (11)

As suggested above, the governing assumption here is that eachdelamination encountered has a transmission coefficient T that isreasonably near the value found for the single delamination in the

scan of the GFRP sample after E ¼ 20 J impact.

A. Khomenko et al. / Composites Part B 107 (2016) 182e191 187

“standard” specimen. However, one might encounter intensityvalues that may not fall in any of these discrete intervals. Thishappens because the incident laser beam is not focused and has afootprint of d~1.5 mm. It was observed that at the boundaries be-tween each delamination and the sample there were smoothtransition regions that were caused by partial attenuation of theincident beam. Fig. 4a through c illustrate this behavior conceptu-ally for regions near the delamination in the “standard” specimen(Fig. 4b) and near the specimen supporting structure (Fig. 4c). Thewidths of the transition regions were estimated to be 2 d~3mm. So,the intermediate values were arbitrarily split equally between theadjacent levels, and the new contour margins were computed asillustrated in Fig. 4d to be [37]:

Fig. 4. (a) Gradual change of transmitted radiation power across delamination boundary ducantilever beam (DCB) sample with a crack that served as the “standard”; (c) OT-scan of thecorresponding to different numbers of delaminations.

CmaxN ¼ ðAmax$T þ AminÞ$TN�1

2; (12)

CminN ¼ ðAmax$T þ AminÞ$TN

2; (13)

where CNmax and CN

min are the upper and lower levels of contourscorresponding to the Nth delamination, Amax and Amin are themaximal and minimal amplitudes of registered radiation trans-mitted through a healthy composite, taken as the amplitudes of thecorresponding detector output voltages, and T is the transmissioncoefficient of the “standard” sample with a single delamination in

e to partial beam scattering through the transition regions; (b) OT-scan of the doublemetal-to-composite interface in the support region; (d) determination of contour levels

Fig. 5. (a) OT-scan of the GFRP sample after E ¼ 20 J impact, using only the healthy region without damage for estimating Amax and Amin; (b) histogram of the corresponding regionand threshold levels for determining maximal and minimal amplitudes of transmitted radiation Amax and Amin.

Fig. 6. (a) Post-processed OT-scan of the healthy sample; (b) post-processed OT-scan of the sample after E ¼ 20 J impact.

Fig. 7. Estimated damage area corresponding to different numbers of delaminations N.

A. Khomenko et al. / Composites Part B 107 (2016) 182e191188

its mid-plane.The values of Amax and Amin were computed for healthy and

impacted GFRP samples by considering the histograms of their

healthy regions only (see Fig. 5a). The voltage outputs of thephotodetector were split into 512 bins, whose mean values weresorted in ascending order. The Amin was selected as the average ofthe first bin containing more than 25 elements. Similarly, the Amax

was assigned the average of the last bin, whose number of elementsexceeded the same threshold. This procedure was applied toremove measurement variations potentially caused by the circuitnoise, tilt of the sample, surface roughness or contamination, andvibration of the fixture during the acquisition of the OT-scans.

3.3. Post-processed OT-scans of GFRP samples

Post-processed OT-scans of healthy and impacted GFRP samplesare shown in Fig. 6 a and b. The color bar of contour plots waspartitioned as per Equations (12) and (13) to highlight the healthyregions of each composite plate and the regions with givennumbers of delaminations. As seen from Fig. 6b, the total delami-nation count was the largest in the middle region of the impactedsample, and it decreased radially from the point of impact. Inaddition, the extents of delaminations were slightly larger alongthe principle directions of the glass fibers, [0/90]4. The results

A. Khomenko et al. / Composites Part B 107 (2016) 182e191 189

demonstrated that it was difficult to determine the differencesbetween contours for a large number of interlaminar defects, inparticular, forN> 4, because themargins became too closely spacedand the overall level of radiation intensity at the photodetectorapproached the noise limit. Hence, whenever the voltage output ofthe photodetector fell below C4

min ¼ 1.58 V, this indicated that Nexceeded four, which is simply denoted as “4þ” in the subsequentfigures.

Fig. 7 provides comparisons of the damage extent as a functionof the number of delaminations. The results demonstrate that re-gions of the GFRP samplewhere the impact created only one or twodelaminations were significantly larger than the regions wherethree and more delaminations appeared.

3.4. OTS validation

The impacted sample (E ¼ 20 J) shown in Fig. 2b was cut with adiamond saw at location x ¼ 37.8 mm along its y-axis (see thecorresponding OT-scan in Fig. 3b). The left half of the sample was

Fig. 8. (a) Cross-section of the GFRP sample (thickness H ¼ 4.8 mm, impact energy E ¼ 20 Jcorresponding OT-linescan; (d) the number of delaminations, N registered by the OTS; (e)processed digital camera images; (f) comparison of results obtained with OTS and digital cmatch are highlighted in purple. (For interpretation of the references to colour in this figur

arbitrarily chosen for determining the delamination contours. Itsnew face created after cutting was saturated with a UV dye pene-trant, and it was left to soak for 30 min. After absorption of the UVdye by the interlaminar defects, the sample was illuminated with aUV lamp in a dark ambience in order to enhance the contrast be-tween the delaminations and the pristine GFRP material in thetransect. The image of the cross-section was taken with a digitalcamera, and it was later converted into a gray-scale format for post-processing in MATLAB with the result shown in Fig. 8a. De-laminations were identified using the two-stage Canny edgedetection algorithm followed by Wiener filtering to smooth theimage and remove the residual artifacts [38]. Applying a hardthreshold to the resulting image effectively converted it into a bi-nary representation, whose high-value pixels determined thedelamination boundaries, and whose zero-value pixels corre-sponded to the pristine GFRP material. Fig. 8b shows the obtainedbinary image merged with the original digital picture of the cross-section area. The delaminations were highlighted as red curves, andthe total number of delaminations at a given scanning position was

); (b) delaminations identified using edge detection and filtering of the image data; (c)the number of delaminations, N along the cross-section length obtained from post-amera imaging wherein the regions in which the numbers of detected delaminationse legend, the reader is referred to the web version of this article.)

A. Khomenko et al. / Composites Part B 107 (2016) 182e191190

easily determined by automatically counting the number of curvesthrough the height of the image.

Fig. 8c illustrates the OT-linescan of the corresponding crosssection of the GFRP sample. In this figure, the curve is partitionedbased on the voltage levels determined from Equations (12) and(13). Thereby, if the total number of delaminations N in the cross-section changes, this is highlighted with a different color inaccordance with the discrete color map on the right hand side.

The total number of delaminations along the cross-section of theGFRP sample, determined from the OT-linescan and the post-processed digital image, are shown in Fig. 8d and e, respectively.Both plots are well aligned along the x-axis, which indicates thatthe OTS technique accurately determines the outer margins of theimpact damage. In addition, one of the characteristic features of thescanned sample is that only two delaminations appear in themiddle of the cross-section, as seen from Fig. 8b. This trend is verywell captured by the OTS system. In Fig. 8f both results are plottedon top of each other, and the regions in which the numbers ofdelaminations detected by the two methods match are highlightedin purple. As seen, the results obtained with OTS and the digitalcamera match well, differing at most by one delamination. Thisdifference can be explained by the fairly large transition region ofthe laser beam (2 d z 3 mm) and light scattering previously dis-cussed. At the same time, the digital image of the cross sectionshows the distribution of the impact damage only in two di-mensions. However, the damage is three-dimensional, and if thenumber of delaminations in the plane parallel to the transect of thesample is not constant within the footprint of the laser beam, theamount of the received radiation at the downstream photodetectorwould be affected. A simple way to improve the agreement and toenhance the resolution would be to reduce the beam diameter.

4. Conclusions

This paper presented optical transmission scanning (OTS) as avaluable technique for quantitative NDE of GFRP composites. InOTS, structural damage is detected and evaluated by measuring theamount of radiation transmitted through the test samples. OT-scans are rapidly acquired without any couplants and do notrequire extensive processing. Moreover, OTS can be readily appliedfor inspection of thick GFRP structures. A special post-processingtechnique for raw OT-scans was developed for evaluation of thedamage introduced in GFRP composite by low velocity impacts. Abasic prototype of the OTS system was able to accurately identifythe total number of delaminations, up to a maximum of four, in thecross-section of the GFRP sample at each scan point. If more thanfour delaminations were encountered along a transect, the exactnumber could not be ascertained with basic OTS system. Additionaluseful information about the severity of impact damage was ob-tained by measuring the extent of each detected delamination. TheOTS and advanced image processing algorithm were experimen-tally validated by comparing OT-linescans with the observed dis-tribution of delaminations in the cross-section of GFRP samplesafter cutting themwith a diamond saw. The presented OTS setup isa promising tool, which lays groundwork for cost-effective, non-contact, rapid, and quantitative NDE of GFRP composite structures.

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